This invention relates to the field of semiconductor transceivers, and more particularly relates to vertical cavity surface emitting laser and resonant cavity photodetector duplex transceivers.
Conventional semiconductor lasers have found widespread use in modern technology as the light source of choice for various devices, e.g., communication systems, compact disc players, and so on. For many of these applications, a semiconductor laser is coupled to a semiconductor receiver (e.g. photodiode) through a fiber optic link or even free space. This configuration may provide a high speed communication path.
A typical semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap, low refractive index layers. The low bandgap layer is termed the "active layer", and the bandgap and refractive index differences serve to confine both charge carriers and optical energy to the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. The cladding layers have opposite conductivity types and when current is passed through the structure, electrons and holes combine in the active layer to generate light.
Several types of surface emitting lasers have been developed. One such laser of special promise is termed a "vertical cavity surface emitting laser" (VCSEL). (See, for example, "Surface-emitting microlasers for photonic switching and interchip connections", Optical Engineering, 29, pp. 210-214, March 1990, for a description of this laser. For other examples, note U.S. Pat. No. 5,115,442, by Yong H. Lee et al., issued May 19, 1992, and entitled "Top-emitting Surface Emitting Laser Structures", which is hereby incorporated by reference, and U.S. Pat. No. 5,475,701, issued on Dec. 12, 1995 to Mary K. Hibbs-Brenner, and entitled "Integrated Laser Power Monitor", which is hereby incorporated by reference. Also, see "Top-surface-emitting GaAs four-quantum-well lasers emitting at 0.85 .mu.m", Electronics Letters, 26, pp. 710-711, May 24, 1990.)
Vertical Cavity Surface Emitting Lasers offer numerous performance and potential producibility advantages over conventional edge emitting lasers. These include many benefits associated with their geometry, such as amenability to one- and two-dimensional arrays, wafer-level qualification, and desirable beam characteristics, typically circularly-symmetric low-divergence beams.
VCSELs typically have an active region with bulk or one or more quantum well layers. On opposite sides of the active region are mirror stacks which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength (in the medium) of interest thereby forming the mirrors for the laser cavity. There are opposite conductivity type regions on opposite sides of the active region, and the laser is typically turned on and off by varying the current through the active region.
Typical resonant cavity photodetectors (RCPDs) may be constructed similar to a VCSEL, but may operated in a reverse bias mode. A resonant cavity photodetector may be more efficient than a standard photodiode because the light that enters the cavity, through one of the mirrors, may be reflected through the active region many times. The light may thus be reflected between the mirror stacks until the light is either absorbed by the active region or until it escapes through one of the mirror stacks. Because the mirror stacks are typically highly reflective near resonance, most of the light is eventually absorbed by the active region, thereby causing electron/hole pairs to be generated therein.
As indicated above, a resonant cavity photodetector typically has a reverse bias applied thereto. The reverse bias may sweep the electron/hole pairs from the active region and to the surrounding mirror stacks. The electron/hole pairs may then be collected by opposing terminals, and a photocurrent may result. This photocurrent may be provided to a receiver circuit.
Prior art VCSEL/RCPD transceivers include at least two separate integrated circuits; one that includes vertical cavity surface emitting lasers and another that includes resonant cavity photodetectors. The VCSEL chip typically includes vertical cavity surface emitting laser devices or arrays. The RCPD chip may include a single or an array of resonant cavity photodetectors, wherein each of the RCPDs may be fabricated to have a relatively broad absorption band. The VCSEL chip may or may not include driver circuitry and the RCPD chip may or may not include receiver circuitry.
This approach suffers from a number of limitations, some of which are described below. First, by having two separate chips, the cost of the optical transceiver may be increased. That is, each chip must typically be separately packaged or inserted into a multi-chip module. Further, interconnections may have to be supplied between the transmitter chip and the receiver chip, or between either of these chips and corresponding driver circuitry. This may require a number of wire bonds of the like, which may reduce the performance of the system, and may add additional assembly expense. In addition, optical communication within a single chip, via waveguides or the like, may be prohibited, and bi-directional optical communication between two chips may be limited or complicated.
In addition to the above, the VCSEL chip and RCPD chip are typically fabricated on at least two separate wafers, and likely two separate runs. Because of fabrication tolerances and other factors which may vary between wafers, the performance characteristics of the VCSEL and RCPD devices may not be sufficiently matched. Thus, it may be difficult to identify a vertical cavity surface emitting laser and a resonant cavity photodetector that have similar temperature and wavelength characteristics, particularly when an entire array of devices must be matched. To compensate for these effects, the absorption band of the resonant cavity photodetectors may have to be increased, which may decrease the overall efficiency and performance thereof.